During the transition from uterine to extrauterine life a remarkable change occurs in fetal pulmonary blood flow and function. All of the factors responsible for the high resistance to blood flow in the fluid filled fetal lung and the relatively low resistance of the newly ventilated neonatal lung have yet to be clarified. The overall objective of this proposed research is to gain a clearer understanding of these factors with emphasis on the site within the pulmonary vasculature at which they act. The pulmonary vasculature of fetal and neonatal goats will be segmentally analyzed in terms of a Starling resistor model. The model allows for the determination of the resistance of the pulmonary vasculature proximal (upstream) to the point of the Starling resistor within the vasculature (Rp), the resistance distal (downstream) to the point of the Starling resistor (Rd) and the pressure surrounding the Starling resistor (Ps). Based on this model, the following will be studied: (1) the effect of vasoactive drugs and diuretics on Rp, Rd, and Ps; (2) the effect of blood gases and pH on Rp, Rd, and Ps; (3) the relationship between fetal and neonatal age and Rp, Rd, and Ps and the effects of ventilation on these parameters at various fetal ages; (4) the effect of lung expansion and alveolar pressure on Rp, Rd, and Ps in fetal and neonatal animals; (5) the site of the Starling resistor within the pulmonary vessels; (6) the relationship between Ps and the intercept of the linear portion of pulmonary pressure-flow curves on the pressure axis; (7) a postulated mechanism for the production of fluid by the fetal lung and the rapid removal of fluid from the alveoli when ventilated; (8) the relationship between Rp, Rd, and Ps and the microscopic anatomy and pathology of the lung in which the above studies are carried out.